1. Technical Field
The invention relates to a record carrier comprising a servo track indicating an information track intended for recording information blocks represented by marks having lengths expressed in channel bits, which servo track has a periodic variation of a physical parameter, which periodic variation is modulated for encoding record carrier information.
The invention further relates to a recording and/or playback device comprising means for writing and/or reading information blocks represented by marks having lengths expressed in channel bits in an information track on the record carrier, which device comprises means for scanning the servo track and retrieving the record carrier information.
The invention further relates to a method for manufacturing the record carrier.
2. Related Art
A record carrier and device of the type defined in the opening paragraph, for reading and/or writing information, are known from U.S. Pat. No. 4,901,300 to Van Der Zande, et al. The information is encoded into an information signal which includes time codes and may be subdivided in accordance with these time codes into information blocks, the time codes being used as addresses such as with Compact Disc Read Only Memory (CD-ROM). The record carrier has a servo track, usually called pregroove, for causing servo signals to be generated when scanning the track. A physical parameter, e.g. the radial position, of the pregroove periodically varies, constituting a so-called wobble. During the scanning of the track, this wobble leads to a variation of the servo signals. The variation is modulated by record carrier information, e.g. synchronization symbols and encoded position information, which position information indicates the absolute length of the track from the start of the track. During recording, the position of the information blocks is synchronized, as much as possible, with the synchronization symbols, so that the information blocks are written on the record carrier at a positions corresponding to their addresses.
A problem of such a system is that the pregroove wobble signal is modulated with a relatively low frequency, and thus it is hard to derive therefrom, with great accuracy and little delay, the record carrier information, e.g. the position of the read/write head, or the instants of occurrence of the synchronization symbols. In addition, the pregroove wobble signal is modulated with little intensity and, therefore, is sensitive to disc flaws.
It is an object of the invention, inter alia, to provide a record carrier and a device in which the record carrier information can be determined in a reliable, rapid and accurate manner.
According to a first aspect of the invention a record carrier as defined in the opening paragraph is characterized in that said modulation is a bi-phase modulation, in which a data bit of the record carrier information is encoded by a first predetermined number of variations of a first phase, followed by the same number of variations of a second phase inverse to the first phase. According to the invention, the recording and/or playback device as described in the opening paragraph is characterized in that the device comprises bi-phase demodulation means for retrieving data bits of the record carrier information from a first predetermined number of variations of a first phase, followed by the same number of variations of a second phase inverse to the first phase. This has the effect, that data bits can be detected independently from their value from a same number of inverted and non-inverted periodic variations. Disturbances like offset, asymmetry or cross talk can be compensated for by combining the detection signals from the inverted and non-inverted variations, (e.g. by integration). This has the advantage, that the detection has the same reliability for bits having the value 0 or 1. When compared to modulating in a single phase (e.g., invert 4 wobbles for bit=1), the bi-phase modulation has the advantage that the total number of inverted wobbles is always the same (irrespective of the data), and equal to the average number of wobbles inverted for single phase,. In a worst case situation for single phase modulation (i.e., for a series of bits=1) twice as many wobbles would be inverted. A further advantage is, that disturbance in detection of the physical location of the periodic variations, which may be used for positioning the marks when recording, is accurate, because inverted variations in a neighboring track only have a low and predictable influence when compared to random modulated variations such as the frequency modulation in the known method.
An embodiment of the record carrier characterized in that the servo track is subdivided in relatively small parts having said bi-phase modulation and relatively large parts having not-modulated (also called monotone) periodic variations. Relatively large indicates that at least 80% of the servo track has the not-modulated periodic variations, and preferably at least 90%. This has the advantage that detection of the periodic variations for positioning information blocks is not disturbed by modulation in a substantial part of the track.
A further embodiment of the record carrier is characterized in that the length of one periodic variation corresponds to a third predetermined number of channel bits. The effect is, that the nominal position of the nth channel bit corresponds exactly to the physical position of the nth periodic variation divided by the third predetermined number, because the bi-phase modulation does not influence the length of the periodic variations. This has the advantage, that the recording location can be synchronized to the periodic variations. A relatively small third predetermined number of channel bits per periodic variation allows a high accuracy of positioning. In particular, 32 is a suitable number, because this is sufficiently longer than the longest mark used in common channel coding, which are smaller than 16 channel bits, and allows easy address calculation in a binary system.
Further embodiments of the method, devices and record carrier according to the invention are given in the dependent claims.